This submission looks remarkably like the Baseball slide armbar but the entry, weight distribution and the spacing differs slightly.
For this move I like to dominate my opponents arm with an overhook. This leads to a much more powerful submission and has the added benefit of being very secure.
The rotation of the foot should be a very quick transition. There should be very little warning that you are about to transition into a submission until you are falling back. This, unlike the Baseball Slide Armbar, does allow you to start falling back before swinging the leg over the head.
Although this does not allow for the same level of domination it can be very quick and lead to some incredibly fast taps. Take note that with the arm behind the armpit you CAN NOT lay your back on the floor while practicing this move. With that in mind, please practice this as I do in the video (without the arm behind the shoulder) UNLESS you have good body control and can stop your body from laying flat on the ground.